Diesel engines enjoy an advantage over gasoline engines in that the diesel engines are much more fuel efficient than gasoline engines. It is well known that the gaseous waste products, hydrocarbons, carbon monoxide and nitrogen oxides, from gasoline engines pose a serious health problem to the population at large. In addition to these gaseous pollutants, diesel engines also emit "soot" particles comprising carbonaceous solids containing adsorbed hydrocarbons and inorganic compounds or very fine droplets of condensate or a conglomerate of the two "particulates". The "particulates" referred to herein as "diesel soot" are particularly rich in condensed polynuclear hydrocarbons, some of which have been found to be carcinogenic. Owing to these factors, the United States Environmental Protection Agency has promulgated strict standards to minimize the discharge of diesel soot from automotive sources into the atmosphere. Additionally, California also has enacted regulations regarding emission of diesel soot from stationary sources.
Several approaches have been proposed to try to solve the diesel emission problem. Among these are: (1) electrostatic precipitators; (2) paper filters; (3) ceramic filters; (4) metal mesh filters; and (5) engine modifications. Electrostatic precipitators are too bulky and require too much energy to operate and are therefore impractical. Similarly, paper filters require frequent replacement and are extremely bulky. Engine modifications are capable of reducing the soot emissions, but not to the point where all vehicles can meet all the emission standards. The reason for this is that modifications which reduce the soot emissions generally increase the nitrogen oxides emissions or reduce the practical operation of the engine.
Ceramic and metallic filters have proven to be the best technology available to deal with this problem. The literature also shows that ceramic filters are preferred over metallic filters because the ceramic filters are apparently more durable. Ceramic filters have been described in the prior art and can be divided into two categories: (1) foam type and (2) honeycomb wall-flow type. Ceramic foam filters have been described in U.S. Pat. No. 4,083,905 and Society of Automotive Engineering Paper #830082. This type of filter is prepared by depositing a ceramic material onto an organic sponge and sintering said sponge at a high temperature to burn out the organic sponge material.
The honeycomb wall-flow type filters are very similar to the honeycomb substrates used as catalyst structural supports for gasoline engine pollution control applications, except that alternate flow channels are closed on each face of the substrate. The channels are plugged in such a manner that a channel open on one face is closed at the opposite face. Such filters are called "wall-flow filters" because the exhaust flows down a channel and must go through the walls of the channel which are macroporous in order to exit. These filters are described in U.S. Pat. Nos. 4,329,162, 4,340,403, 4,364,760 and 4,423,090. These wall-flow filters have been used more extensively than the foam filters because the wall-flow filters more efficiently trap the diesel soot.
The biggest drawback to these filters is that the diesel soot accumulates, clogging the filter, thereby causing an undesirable backpressure on the engine. The reason for this accumulation is that diesel soot ignites at about 650.degree. C., but the maximum exhaust temperature in a diesel vehicle is only about 300.degree.-400.degree. C. Therefore, the diesel soot continues to build up and causes excessive back pressure on the engine which results in a decrease in fuel economy and eventually may cause damage to the engine. To alleviate this problem, the diesel soot must be burned off.
There are two ways known in the art to burn or ignite the diesel soot collected on these filter traps. First, an external means of heat can be applied to the filter so that the temperature of the filter is raised high enough to initiate soot combustion. Second, the filter can be coated with a catalytic element that will lower the required combustion temperature of the diesel soot.
The first approach has many disadvantages including: (1) reduction of fuel economy; (2) complexity of the control system; and (3) reliability of the overall system. In contrast, the second approach is much simpler and more reliable. The major problem with the second approach is developing a catalytic composite which lowers the ignition temperature of the diesel soot so that combustion of the diesel soot occurs during normal operating conditions.
In addition to igniting the diesel soot, most catalytic composites will also convert the sulfur oxides in the exhaust to sulfates or sulfuric acid. Although this problem is present in gasoline powered engines, it is particularly troublesome in diesel applications for two reasons. First, diesel fuel typically contains at least ten times more sulfur compounds than gasoline fuel. Second, the low temperature of the diesel exhaust facilitates the production and storage of sulfates and sulfuric acid which contribute to the particulate emissions during high temperature modes, like regenerations.
It is recognized that noble metals, especially platinum, can oxidize both gaseous hydrocarbon and soot particles. Additionally, platinum promotes the conversion of sulfur oxides to sulfates. U.S. Pat. No. 4,617,289 claims to solve this problem by adding large amounts of vanadium oxide (V.sub.2 O.sub.5) to minimize the sulfate formation.
The prior art teaches that platinum is not a preferred metal for combusting diesel soot unless a promoter is used. For example, see U.S. Pat. No. 4,617,289 and references therein. Additionally, other patents teach that platinum should be used only for converting the gaseous hydrocarbon and other elements such as chromium, silver, etc. are best for igniting the soot. For example, U.S. Pat. No. 4,303,552 teaches the use of platinum and a bulk component selected from the group consisting of an element of the first transition series, silver and hafnium deposited on an inorganic oxide, preferably alumina.
Further, U.S. Pat. Nos. 4,515,758 and 4,588,707 teach the use of rhenium plus substances such as lithium oxide, copper chloride, vanadium oxide and optionally a noble metal. Again the noble metal is used only for treating the gaseous emissions. These patents also teach that the soot burning elements, i.e. rhenium, lithium oxide, etc. are deposited on an inorganic oxide support such as alumina, titania, etc.
It is also known that supports such as titania or zirconia have sulfur resistant properties. For example, see U.S. Pat. No. 4,350,613. However, none of the patents cited teach the use of a composite comprising platinum or other noble metals on titania to reduce the ignition temperature of diesel soot.
The present invention provides a process to reduce the ignition temperature of diesel soot comprising contacting a hot exhaust from a diesel engine with a catalytic composite comprising a particulate filter having deposited thereon a sulfur resistant refractory inorganic oxide such as titania, zirconia, etc. and dispersing on said oxide at least one catalytic element selected from the group consisting of Pt, Pd and Rh. The present invention shows unexpected results in lowering the ignition temperature of diesel soot.